Fix tests

Fixes gh-3832

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/handler/NoUnboundElementsBindHandler.java

@@ -77,7 +77,7 @@ public class NoUnboundElementsBindHandler extends AbstractBindHandler {
 			BindContext context) {
 		Set<ConfigurationProperty> unbound = new TreeSet<>();
 		for (ConfigurationPropertySource source : context.getSources()) {
-			if (this.filter.apply(source)) {
+			if (source instanceof IterableConfigurationPropertySource && this.filter.apply(source)) {
 				collectUnbound(name, unbound,
 						(IterableConfigurationPropertySource) source);
 			}

spring-boot/src/test/java/org/springframework/boot/context/properties/ConfigurationPropertiesBindingPostProcessorTests.java

@@ -48,6 +48,7 @@ import org.springframework.context.support.PropertySourcesPlaceholderConfigurer;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.MapPropertySource;
 import org.springframework.core.env.MutablePropertySources;
+import org.springframework.core.env.StandardEnvironment;
 import org.springframework.core.env.SystemEnvironmentPropertySource;
 import org.springframework.mock.env.MockEnvironment;
 import org.springframework.test.context.support.TestPropertySourceUtils;
@@ -426,7 +427,7 @@ public class ConfigurationPropertiesBindingPostProcessorTests {
 		MutablePropertySources propertySources = env.getPropertySources();
 		propertySources.addFirst(new MapPropertySource("test",
 				Collections.singletonMap("com.example.foo", 5)));
-		propertySources.addLast(new SystemEnvironmentPropertySource("system",
+		propertySources.addLast(new SystemEnvironmentPropertySource(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME,
 				Collections.singletonMap("COM_EXAMPLE_OTHER", "10")));
 		this.context.register(TestConfiguration.class);
 		this.context.refresh();